TPA3128D2_17 Datasheet, PDF (20/38 Pages) Texas Instruments – 2x30-W, 2x15-W Class-D Amplifier With Low Idle Power Dissipation

TPA3128D2, TPA3129D2
SLOS941B – MAY 2016 – REVISED JUNE 2017
www.ti.com
7.3.13 Efficiency: LC Filter Required with the Traditional Class-D Modulation Scheme
The main reason that the traditional class-D amplifier-based on AD modulation requires an output filter is that the
switching waveform results in maximum current flow. This causes more loss in the load, which causes lower
efficiency. The ripple current is large for the traditional modulation scheme, because the ripple current is
proportional to voltage multiplied by the time at that voltage. The differential voltage swing is 2 × VCC, and the
time at each voltage is half the period for the traditional modulation scheme. An ideal LC filter is required to store
the ripple current from each half cycle for the next half cycle, while any resistance causes power dissipation. The
speaker is both resistive and reactive, whereas an LC filter is almost purely reactive.
The TPA3128D2 and TPA3129D2 modulation schemes have little loss in the load without a filter because the
pulses are short and the change in voltage is VCC instead of 2 × VCC. As the output power increases, the
pulses widen, making the ripple current larger. Ripple current could be filtered with an LC filter for increased
efficiency, but for most applications the filter is not required.
An LC filter with a cutoff frequency less than the class-D switching frequency allows the switching current to flow
through the filter instead of the load. The filter has less resistance but higher impedance at the switching
frequency than the speaker, which results in less power dissipation, therefore increasing efficiency.
7.3.14 Ferrite Bead Filter Considerations
Using the Advanced Emissions Suppression Technology in the TPA3128D2 and TPA3129D2 amplifiers, a high
efficiency class-D audio amplifier can be designed while minimizing interference to surrounding circuits.
Designing the amplifier can also be accomplished with only a low-cost ferrite bead filter. In this case the user
must carefully select the ferrite bead used in the filter. One important aspect of the ferrite bead selection is the
type of material used in the ferrite bead. Not all ferrite material is alike, therefore the user must select a material
that is effective in the 10-MHz to 100-MHz range which is key to the operation of the class-D amplifier. Many of
the specifications regulating consumer electronics have emissions limits as low as 30 MHz. The ferrite bead filter
should be used to block radiation in the 30-MHz and above range from appearing on the speaker wires and the
power supply lines which are good antennas for these signals. The impedance of the ferrite bead can be used
along with a small capacitor with a value in the range of 1000 pF to reduce the frequency spectrum of the signal
to an acceptable level. For best performance, the resonant frequency of the ferrite bead/ capacitor filter should
be less than 10 MHz.
Also, the ferrite bead must be large enough to maintain its impedance at the peak currents expected for the
amplifier. Some ferrite bead manufacturers specify the bead impedance at a variety of current levels. In this case
the user can make sure the ferrite bead maintains an adequate amount of impedance at the peak current the
amplifier will see. If these specifications are not available, the device can also estimate the bead current handling
capability by measuring the resonant frequency of the filter output at low power and at maximum power. A
change of resonant frequency of less than fifty percent under this condition is desirable. Examples of ferrite
beads which have been tested and work well with the TPA3136D2 can be seen in the TPA3136D2EVM user
guide SLOU444.
A high quality ceramic capacitor is also required for the ferrite bead filter. A low ESR capacitor with good
temperature and voltage characteristics will work best.
Additional EMC improvements may be obtained by adding snubber networks from each of the class-D outputs to
ground. Suggested values for a simple RC series snubber network would be 18 Ω in series with a 330 pF
capacitor although design of the snubber network is specific to every application and must be designed taking
into account the parasitic reactance of the printed circuit board as well as the audio amp. Take care to evaluate
the stress on the component in the snubber network especially if the amp is running at high PVCC. Also, make
sure the layout of the snubber network is tight and returns directly to the GND pins on the IC.
Figure 31 and Figure 32 are TPA3128D2 EN55022 Radiated Emissions results uses TPA3128D2EVM with 8-Ω
speakers.
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